Friction Reduction for Engine Components

ABSTRACT

The present invention relates to techniques for lowering friction between moving surfaces of, for example, an internal combustion engine. Friction reduction is achieved by adding texture modifications to surfaces that come in contact with each other. Texture modifications that reduce friction in accordance with the present invention include dimples of varying geometries and depths ion the surfaces of components. The present invention also relates to the lubrication technique for applying the texture to the surfaces. In another embodiment, the patterned soft mask is applied onto a large surface (flat or curved including cylindrical rollers surfaces) to be followed by electrochemical etching to imprint the textures onto the component And, in another embodiment, a diamond-like-carbon (DLC) film may be applied to the turbine component to also reduce friction.

This application claims priority to provisional U.S. application Nos.61/445,503 and 61/445,507, both filed on Feb. 22, 2011. The entiredisclosures of both such applications are incorporated herein byreference.

This invention was made with government support under Department ofEnergy Prime Contract No. DOE/NETL DE-AC26-04NT41817-520.01.04 viaSubcontract No RDS 41817M3459 awarded to George Washington Universityfrom Research and Development Solutions, LLC. The government has certainrights in the invention.

BACKGROUND OF INVENTION

As crude oil prices soar, energy efficiency in the transportation sectorhas become especially important. Accordingly, there a very high demandfor modern engines and drive trains to improve fuel economy and energyefficiency by reducing frictional losses at various contact interfaces.The present invention may have the benefit of improving fuel economy andenergy efficiency by reducing friction between engine components undervarious operating conditions.

SUMMARY OF THE INVENTION

One aspect of this invention relates to incorporating surface texturedesigns onto engine component surfaces, which may significantly reducefrictional losses for high-contact pressure regions. Previously, surfacetexturing had only been used commercially in applications involving twoflat surfaces or two conformal surfaces. Specific applications with suchsurfaces have included, for example, water pump seals in automobiles andseals in pumps. Those applications generally involve high speed,low-load operating conditions.

FIG. 1 shows the various combinations of speed and load operatingconditions. Prior art texture patterns have been shown to be effectivein friction reduction under high speed, low-load conditions (Regime I)but lose effectiveness under other load and speed ranges (in fact,friction rises due to surface contacts and the dimple edge stresses asif the surface becomes rougher) (Regime II and Regime III).

The basic principles of surface texture designs as reflected in FIG. 1are summarized as follows:

(Regime I) hydrodynamic lubrication regime (“HD”) (high-speed andlow-load)—the effects of textured surfaces are a) enhanced hydrodynamiclubrication with dimples or grooves; b) cavitational pressure liftmechanism under certain conditions; and c) reverse flow induced bydimples or grooves.

(Regime II) elastohydrodynamic lubrication regime (“EHL”) (increasingload from HD regime)—a) not all dimple sizes and shapes would reducefriction (they actually increase friction if not implemented properly).The dimples have to induce squeezed film lubrication (wedge effect) orpartial kinetic hydrostatic lift lubrication mechanism to effectfriction reduction. Therefore, the angle of the dimple edge with respectto horizontal plane, bottom shape, dimple spacing in both x and ydirections are important, and potential cavitation lift could also beone of the mechanisms to effect friction reduction.

(Regime III) boundary lubrication regime (much lower speed and higherload, so the two sliding surfaces touch one another)—the dimplesoperating in this regime require hydrostatic pressure to be generated bythe trapped lubricant to enable the onset of hydrodynamic filmlubrication mechanism.

There have traditionally been two common recognitions in the texturingcommunity based on experimental results: 1) only one of the two slidingsurfaces should be textured; 2) texture only works in conformalcontacting surfaces (such as journal beatings, seals, but will not workon curved surface sliding against another curve surface). The presentinvention circumvents these two restrictions by selecting the shape andsizing the dimples smaller than the contact width to function properlyunder these conditions.

Additionally, significant technical barriers for introducing textures toengines have previously existed relating to the lack of low-costfabrication techniques, which could allow for the application oftextures over large areas of surface including, for example, largeautomotive and diesel engine components, which are made of tool steel orbearing steel, sometimes with additional surface treatment to enhancedurability. Accordingly, another aspect of this invention relates to thesurface texture fabrication procedure. Conventional microlithographyuses rigid masks (typically glass or Mylar) (usually at severalmillimeter dimensions) and is not capable of fabricating textures oncurved surfaces due to the need for a UV exposure processing step.Alternative techniques such as embossing, nanomechanical scratching,nanoprinting, and laser ablation techniques 1) are very expensive; 2)are time consuming; 3) potentially damage the surface resulting infatigue strength degradation; and 4) are not capable of fabricatingcomplex mixed shape feature patterns in one processing step. Therefore,one of the crucial barriers to the potential widespread application ofsurface texturing to engine components has been the lack of large areaflexible masks that are capable of wrapping around large automotive anddiesel engine components for lithography and electrochemical etching.

Another aspect of this invention attempts to resolve the wear problem oftextured surface running in occasional boundary lubrication regime byadding a tough thin protective diamond dike-carbon film (“DLC”),specially formulated and processed to have unique combination ofphysical, chemical, and nanomechanical properties for this application.DLC films can have many variations and traditional DLC films that havebeen tested do not work under this application. DLC films generally havelower reactivity towards traditional lubricants (containing typicalphosphorus antiwear additives such as zine dialkyl dithiophosphate ortrecreyl phosphates). According to an embodiment of this invention, abonded chemical film contains chemistry that reacts with the DLC film sothat the DLC film carries its own lubricant chemistry to make thecombination highly durable. At the same time, DLC with bonded chemicalfilms demonstrate highly effective friction reduction in enginecomponents and make the surface textures long lasting. This aspect ofthe invention can also be used to protect surfaces of non-automotivecomponents as well.

DLC, as used herein, is a generic term describing a class ofcompositions. There can be thousands of diamond-like carbon films withdiffering and varying properties and thicknesses, processing techniques(different gaseous atmospheres during deposition), and differing degreesof “filtering” of raw materials vapors and particles. The properties ofthe DLC film depend strongly on processing methods, processingconditions, the gases used in the sputtering process, and the thicknessof the film. DLC films include nanocrystalline diamond particlesdispersed in an amorphous carbon matrix. The number of diamond particlesin a particular DLC film can be measured using Laser Raman Spectroscopy,as SP³ peak, or any other suitable method. The graphitic matrix isrepresented by the SP² peak. The term SP³/SP² reflects the hardness andtoughness of the film. Not all DLC films can protect textured surfaces.An embodiment of this invention relates to a DLC film that can be usedto protect textured surfaces.

The present invention relates to techniques for lowering frictionbetween moving surfaces of, for example, in an internal combustionengine. The present invention generally is directed to reducing frictionbetween parts of an internal combustion engine by introducing texturemodifications in the surfaces that come in contact with each other. In apreferred embodiment, these surfaces are engine piston rings, cylinderliners, and their respective pins and bearings. In one embodiment, thesurface textures are depressions of specific sizes, shapes, andarrangements. In another embodiment, the surface textures can be ofdifferent sized textural shapes that vary depending on the engineoperational load and speed.

In one aspect of the invention, a texture pattern is implemented thatreduces friction as components move from low-load, high-speed conditionsto high-load, low-speed conditions. In this aspect of the invention,dimples act as a wedge, which effectively lifts the surface and causes adecrease in viscous shearing and a corresponding reduction in friction.

Another aspect of the invention relates to surface texture design withthe mixture of different surface features shapes and sizes. This type ofmixed shape texturing combines textures designed for low-load,high-speed operating conditions and the textures that are designed forhigh-load, low-speed operations. This type of texture arrangement may bereferred to as a multiscale texture. By using elliptical-shaped(typically smaller and deeper dimples), and circular-shaped dimples(typically large size and shallower) the result is enhanced frictionreduction for an engine component experiencing variable loads and speedsin its operation (such as a ring liner, cam lifter, etc.). Thisarrangement may allow for the reduction of friction in engine componentsduring operation when the component experiencing various load and speedranges.

Another aspect of the invention relates to the surface texturefabrication technique. Such technique may use acid (or alkaline)resistant flexible (soft) large area masks on large area steel enginecomponents with curved surfaces, for example, internal combustion enginepiston rings and cylinder liners, cams, lifters, rollers, roller pins,etc. In one embodiment of the invention, the surface textural featuresand associated pattern are created on the large size of elastomericpolymer film (soft mask) with dimension accuracy. In another embodiment,the patterned soft mask is applied onto a large surface (flat or curvedincluding cylindrical rollers surfaces) to be followed byelectrochemical etching to imprint the textures onto the steel enginecomponent.

In one aspect of this invention, a pattern may be created on a softplastic mask. Starting out with a clean silicon wafer, a self-assembledmonolayer (“SAM”) of hydrophobic molecules (to prevent sticking) aredeposited on the wafer; this is followed by placing a pure silver (Ag)film on top of SAM. A photolithographic process is used to produce thedesired texture pattern on the silver film. The patterned silver film isthen coated with a hydrophilic SAM layer. On top of the SAM layer, apoly dimethylsiloxane (“PDMS”) film is deposited using spin-coating.After baking, the patterned PDMS, i.e., the soft mask, is peeled offfrom the Si surface.

In another aspect of this invention, the soft mask may be directlyapplied onto the engine components. The size of the soft mask is onlylimited by the size of the silicon wafer available (for example, andwithout limitation, a six-inch, 12-inch, or 18 inch-diameter siliconwafer may be used). The soft mask is very flexible with a typicalthickness in the range of 0.1-1.0 mm. The texture pattern on this softmask may be precisely transferred onto the components without thelimitation of size and curvature. The surface textures can then beetched into the surface using conventional electrochemical etchingtechniques. The technique of using large size of patterned soft maskovercomes the technical barriers in applying surface texture to enginecomponents.

Another aspect of the present invention relates to surface layers forprotecting surface texture from wear and reducing friction betweenmoving surfaces, such as in an internal combustion engine component.This aspect of the invention is directed to protecting the surfacetextures by introducing thin film or coatings under high-load, low-speedconditions where wear may damage the textures. In one embodiment, theprotective layers include wear resistant coating. In another embodiment,the protective layers include bonded chemical film on top of the wearresistant coating to provide long term durability of the texturedsurface.

In another embodiment of the invention, the engine or machine componentsare first textured to have small dimples on their surface. The dimplesare then coated with a DLC layer. In another embodiment of thisinvention, the DLC layer is further coated with a reactive chemistrywith the DLC layer. (DLC is inert and hence does not react with mostantiwear chemistries, such as zinc dialky dithiophosphates or Trecylphosphates, the two most dominant antiwear additives on the markettoday). This reactive layer will protect the DLC film andsynergistically reduce the friction further.

In another embodiment of this invention, to provide additional impactresistance, common in engine operation or in other applications, afterthe introduction of the reactive chemistry bonded to the enginecomponent surface, an inert molecule is introduced to the surface by dipcoating, such as alkylcyclopentanes, which does not bond with DLCsurface. This is to be followed by creating a polymer canopy on top ofthe mixed molecular assembly. An alkmonomer layer is spin-coated on topof the molecular assembly and UV radiated to form a polymer layer on thesurface. This helps to prevent the evaporation of the reactive chemistryand encapsulate the inert molecule underneath, providing a visco-elasticlayer. This also helps to achieve enhanced durability and reactivity ofthe DLC coating with the friction reducing anti-wear additives in thelubricant. This process effectively makes the film become part of thesurface structure, and, therefore, it will not be leached out by thelubricant used in the engine or machine, and can function independently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows surface texture friction reduction technology as a functionof speed and load.

FIG. 2 shows exemplary surface pattern designs in accordance with anaspect of the present invention.

FIG. 3 contains experimental friction coefficient data of pin-on-disktesting under low load condition of 0.15 MPa.

FIG. 4 contains experimental friction coefficient data of four balltesting under high load condition of 157 MPa.

FIG. 5 contains experimental data of Cameron-Plint reciprocating testsusing step loading procedure for various frequencies.

FIGS. 6A, 6B, 6C illustrate surface texture designs of pure ellipse,pure circle and a mixture of ellipse and circle according to embodimentsof the present invention.

FIG. 7 contains experimental data illustrating the friction reduction ofsurface texture patterns at different frequencies: pure ellipse, purecircle, mixture of ellipse and circle.

FIG. 8 depicts a schematic diagram of soft lithography process inaccordance with an aspect of the present invention.

FIG. 9 depicts a schematic diagram of the soft mask fabrication inaccordance with an aspect of the present invention.

FIG. 10 depicts an optical image showing the peeling of PDMS mask fromSi wafer piece.

FIG. 11A depicts an optical image showing textural figures on Ag film.

FIG. 11B depicts an optical image showing textural figures on PDMS mask.

FIG. 12A depicts an optical image showing a transparent PDMS mask withtextures.

FIG. 12B depicts an optical image showing a texture feature on PDMS maskafter the deposition of Ag film.

FIG. 12C depicts an optical image showing a texture on steel surfaceetched from the soft PDMS mask.

FIG. 13A depicts an optical image showing an overview of the large sixephoto PDMS mask.

FIG. 13B depicts an optical image showing a magnified image of locationA.

FIG. 14A depicts an optical image showing microfracture of a masterpattern.

FIG. 14B depicts an optical image showing textural feature on thefabricated PDMS mask.

FIG. 15A depicts TEM images showing surface texture of circular dimpleson an engine piston ring sample

FIG. 15B depicts TEM images showing surface texture of ellipticaldimples on an engine piston ring sample.

FIG. 16 shows the delamination of hard thin film at the edge of a dimpleafter four ball testing

FIG. 17A shows the morphology of the DLC coatings on a steel substrate

FIG. 17B shows the adhesion of the DLC coatings after indentation byRockwell 150 kgf

FIG. 18 provides the pin (ball) on disk test results showing frictioncharacteristics of chemical films as a function of contact pressures

FIG. 19 depicts lest results of various bonded chemical films undergoingthe cyclic step loading mini-cycles.

FIG. 20 depicts test results illustrating the time to failure for asurface coated with a DLC thin film.

FIG. 21 depicts Stage I test results for a specific combination ofchemistry and textures.

FIG. 22 depicts Stage II durability test results showing time to failuredata for the bonded chemical films.

DETAILED DESCRIPTION OP THE DRAWINGS

The surface texturing in accordance present invention may utilizetextural patterns on a surface of various shapes, sizes, depth,arrangements, and areal density. The various shapes may include, but arenot limited to, circles, ellipses, triangles, parallelograms, andgrooves of various depth, edge angles, and several orientation angleswith respect to the sliding directions. The size of dimples(depressions) may range from few microns up to hundreds of microns. Insome applications, it is advantageous to fabricate multi-scale dimplesor profusions to achieve friction reduction in large components such asmarine diesel engines. The typical depth of depression may be in therange of 1-8 microns. Larger depressions provide enhanced hydrodynamiclubrication at high speed. This permits the large surface features tooperate effectively under high speed. Small depressions provide extralift due to the squeeze of lubricant trapped in the depression. Thispermits the small surface features to operate effectively under lowspeed and high load conditions. The areal density of large size dimplesmay be in the range of 15% to 25%. The areas density of small sizedepressions may be in the range of 5% to 10%. The arrangement of thesedifferent depressions may be arranged in an alternating pattern.However, any suitable arrangement of depressions of the same, similar,or different sizes may be used.

As an example of an embodiment of one aspect of the invention, texturingmay be applied to a piston ring segment in a typical top piston ringslides over a cross-hatched cylinder liner. The cross hatch marks may beof the same order of magnitude in size and shape to dimples with thecross hatch angle of from 17° to 27° with respect to the horizontalplane of the engine cylinder, and the intersecting angle between thecross-hatching marks may be about 35° to 55°. The height of thecross-hatching lines may be about 3 to 10 μm. This may allow the dimpleson the ring surface to work with another “textured” surface.

In an exploratory study, two separate individual patterns wereincorporated onto a surface at the same time: one large surface texturepattern at or below two microns deep; one elliptical dimple patternabove two microns deep. In this study, the presence of both dimplepatterns actually enhanced one and other as the combined patternachieved much lower friction coefficients under both low loads and highloads over a wide speed range.

In another study, deep small dimples were fabricated inside of largedimples. As an initial design, a mixed pattern was used, i.e., dualtextures of small deep dimples and large shallow dimples with built-insmall deep dimples inside large shallow dimples (multiscale texture).This was followed by a totally integrated design (large shallow dimpleswith small deep dimples inside).

The resulting compound textures were evaluated using the test conditionsshown in Table 1, where the test conditions for pin-on-disk and fourball testers are tabulated.

TABLE 1 Test conditions for pin-on-disk and four ball testers Four-ballwear tester Load: 10 Kg  (157 MPa) 20 Kg  (314 MPa) 30 Kg  (470 MPa)Speed: 3500 rpm (1.34 m/s) 3000 rpm (1.15 m/s) 2500 rpm (0.96 m/s) 2000rpm (0.77 m/s) 1500 rpm (0.57 m/s) 1000 rpm (0.38 m/s) 500 rpm (0.19m/s) Pin-on-disk wear tester Load:  5N  (0.15 MPa) 15N  (0.43 MPa) 25N (0.75 MPa) Speed: 18 rpm (0.038 m/s) 36 rpm (0.076 m/s) 57 rpm (0.123m/s) 76 rpm (0.160 m/s) 96 rpm (0.202 m/s) 115 rpm (0.241 m/s) 129 rpm(0.276 m/s) 150 rpm (0.314 m/s)

Examples of various surface texture designs are shown in FIG. 2:polished surface 1; large shallow textures 2; small deep textures 3;mixed large circles with small deep dimples 4; mixed large circles withsmall deep dimples built-in 5 and 6; compound texture one 7; andcompound texture two 8. Differences between compound texture one 7 andcompound texture two 8 are that the area densities of the small, deeptextures are 5% and 1%, respectively, and the area densities of surfacetextures being used are 7% for small deep elliptical dimples, and 17.5%for large shallow dimples. Mixed designs 4-6 are intermediate designswhere both small and large dimples are present; the large dimplescontain small deep dimples at the center.

These surface texture patterns were evaluated in low-load, high-speedconditions (run on the pin-on-disk tester), and high-load, low-speedconditions (run on the four ball tester). The experimental results areshown in FIGS. 3 and 4. FIG. 3 shows the friction reduction comparisonof five texture patterns under low load conditions (0.15 MPa) ran on thepin-on-disk tester. When the texture patterns are integrated into amixture of large and small dimples, further friction reduction isobserved. FIG. 4 illustrates the comparison of friction reductioncharacteristics under high load conditions (157 MPa) using a four balltester. Significant friction reduction (up to 80%) is observed underhigh-load, low-speed conditions. It has been demonstrated that texturalpatterns with a mixture of large and small dimples are effective inreducing friction significantly under both high speed/low load and highload/low speed conditions.

In a study for a diesel engine piston ring-cylinder liner application,two different shapes of dimples were incorporated onto a surface to forma pattern: circular-shaped and elliptical-shaped dimples were arrangedin an array such that alternating of circle and ellipse with definedareal density. The presence of both dimple shapes and sizes actuallyenhances one and other. The combined pattern achieves much lowerfriction coefficients under both low speed and high speed over a wideload range.

Testing on the effect of surface texturing has also been performed usingCameron-Plint reciprocating test rig. The cylinder liner sample is asegment of production cylinder, such that the liner surface consists ofcross-hatching lines. The cross-hatched lines are a form of surfacetexture commonly used in industrial applications. The width of the linersegment is 12 mm and its length is 58 mm. The piston ring segment slidesagainst the liner segment with a stroke length of 12.4 mm. Based on thegeometry of testing setup, the apparent contact pressure is in the rangeof 0.8-6.7 MPa. The equivalent average linear sliding speed ranges from0.05 m/s to 0.6 m/s for the reciprocating frequencies of 2-24 Hz. Whiteoil (ISO 32 grade equivalent) with 2% Tricresylphosphate (TCP) and 1%antioxidant was used as the testing lubricant. This simple treatmentprevented rapid degradation of lubricant without affecting the frictioncharacteristics. For a selected frequency, a step loading procedure wasused to apply loads from 30 N to 240 N with increments of 30 N. The testwas run for five minutes for each load step, and the average frictionalforce was recorded. This procedure was repeated for frequencies from 2Hz to 24 Hz in increments of 2 Hz. The test conditions are listed inTable 2.

TABLE 2 Test conditions for Cameron-Plint test rig Cameron-Plint testrig Load Frequency Speed  30N (0.83 MPa) 2 Hz (0.05 m/s)  60N (1.67 MPa)4 Hz (0.10 m/s)  90N (2.50 MPa) 6 Hz (0.15 m/s) 120N (3.33 MPa) 8 Hz(0.20 m/s) 150N (4.17 MPa) 10 Hz (0.25 m/s) 180N (5.00 MPa) 12 Hz (0.30m/s) 210N (5.83 MPa) 14 Hz (0.35 m/s) 240N (6.67 MPa) 16 Hz (0.40 m/s)18 Hz (0.45 m/s) 20 Hz (0.50 m/s) 22 Hz (0.55 m/s) 24 Hz (0.60 m/s)

One set of the testing results is illustrated in FIG. 5 showing themeasured friction coefficient at 40° C. plotted against the applied loadand frequencies. The three-dimensional plot illustrates four distinctlubrication regimes: first region 51 represents boundary lubricationregime that the friction coefficient is greater than 0.1; second region52 represents the mixed lubrication regime that a typical frictioncoefficient is in the range of 0.07 to 1.0; third region 53 representsthe elastohydrodynamic lubrication regime where the friction coefficientis in the range of 0.04 to 0.07; forth region 54 represents the fullfilm hydrodynamic lubrication regime with a friction coefficient around0.02 to 0.03. The Cameron-Flint reciprocating testing was carried outfor textured piston ring specimens at room temperature using the testprocedure described above. The un-textured piston ring was also testedat room temperature as baseline for comparison purpose.

As an example of testing performed on an embodiment of the presentinvention, in a diesel engine piston ring-cylinder liner application,two different shapes of dimples have been incorporated onto a surface toform a pattern: circular-shape and elliptical-shaped dimples werearranged in an array such that alternating of circle and ellipse withdefined areal density.

The Cameron-Plint reciprocating testing was carried out for texturedpiston ring specimens at room temperature using the test proceduredescribed above. The un-textured piston ring was also tested at roomtemperature as baseline for comparison purpose.

These texture pattern designs are shown in FIG. 6, which shows a patternof ellipses (FIG. 6A), a pattern of circles (FIG. 6B), and a mixture ofellipses and circles (FIG. 6C). It was found that the presence of bothdimple shapes and sizes enhances one and other as the combined patternachieves a much lower friction coefficients under both low speed andhigh speed over a wide load range.

The testing results at frequencies of 24 Hz are shown in FIG. 7. Thebaseline testing used an un-textured piston ring sample running againsta cross-hatched cylinder liner sample. As shown in the figure, a higherfriction coefficient is observed for ellipse and circle texture patternsunder the condition tested. The design incorporating both ellipse andcircle shapes, can reduce friction up to 40%. The mixture design ofincorporating both ellipse and circle shapes is therefore effective infriction reduction for a wide range of loads and frequencies.

The surface texturing described herein may be applied by variety ofmethods; however, as discussed above, the prior art methods have anumber of drawbacks. Accordingly, the present invention may use apatterned elastomeric polymer film (soft mask) to create texturepatterns to be etched onto real engine component parts made of steel. Inone embodiment, this invention relates to a method, or procedure ofcreating patterns onto the soft mask. In another embodiment, the softmask may be directly applied onto the engine component to createpatterns via electrochemical etching. The engine component may have arough, wavy, or curved surface of hundreds of millimeters in size.Electrochemical etching may be used to create textures (arrays ofmicrometer sized dimples of various geometry, shape, and pattern) on theengine component surface.

Soft lithography techniques have been developed to deposit organicmolecular assemblies on clean flat surfaces, such as silicon wafersurface for the construction of micro devices (MEMS or NEMS) for sense,compute, and actuate purposes. However, no known techniques have beendeveloped for making masks for large area texturing and on relative“rough” surfaces such as production engine parts, which have highsurface roughness and waviness. Engine parts materials also have verycomplex surface materials composition, such as multi-layer surface layercomprising of metals, ceramic powders, and eutectic multiphasematerials. Hence, making photoresist (polymeric material) adhere to thesurface, depositing self assembled molecules, and controlling thebonding of organic molecules to such surfaces are extremely difficult.On rough surfaces, the roughness itself produces varying surface energylevels and defect sites: as a result, depositing uniform molecularlayers may not be feasible, making a uniform organic layer (photo-resistdeposition) for proper UV exposure time for photolithographic purposedifficult. All these barriers make such soft masks fabrication forelectrochemical etching of steel parts unavailable until now.

FIG. 8 shows an embodiment of the present invention that involves aprocess to fabricate a soft patterned elastomeric polymer film as a maskto pattern “soft materials” (polymers, gels, and organic molecules) onsurfaces and the use of a PDMS (polydimethylsiloxane) mask.

With reference to FIG. 8, the process may start with a silicon wafer101. Photoresist 103 is deposited onto the silicon wafer 101 at step102. A mask 105 is added at step 104. UV light exposure is applied, themask 105 is removed, and photoresist 103 is dissolved at step 106,leaving the resulting master 107. PDMS 109 is poured onto the master 107at step 108. Curing that may take place at approximately 65° C.,followed by peeling away of the PDMS 109, occurs at step 110. Theresulting PDMS 109 contains embossed microstructures 111.

Since the PDMS mask can conform to irregular surfaces, the technique, intheory, can be applied to curve surfaces. However, soft lithography hasnot been applied to large area lithography or used in subsequentetching. Engine components require sixes of hundreds of millimeters andrequire subsequent UV lithography and chemical etching steps. Also PDMSmaterial is transparent to ultraviolet light; therefore, the PDMS maskcannot be used for UV lithography and etching steps.

In accordance with this invention, photo-lithography may be combinedwith a thin film deposition technique, and soft-lithography to transfermicro-patterns from a Si wafer coated with Ag film onto a softtransparent PDMS film. To separate the PDMS film from the siliconsubstrate, a self-assembled monolayer (“SAM”) has to be deposited on thefilm to ensure that the film can be peeled off easily.

FIG. 9 illustrates an exemplary fabrication procedure. A hydrophobic SAMlayer may be created on the Si wafer (121). On top of the SAM layer, a100 nm Ag film may be deposited (122) and spin-coated with photoresistmaterials (123). The silver film-covered silicon wafer then may gothrough UV lithography (124) and chemical etching (125) steps, followedby photoresist removal (126) to produce the desired texture pattern.Then a hydrophilic SAM layer may be deposited (127). A PDMS mixture maythen be spin coated (128) onto the hydrophilic SAM layer covered Sisurface with Ag pattern and baked at, for example, 65-70° C. for 3 hours(129). Finally, the cured PDMS film may be peeled off from the Sisurface (130) leaving a soft mask (131). Using this technique, largesize masks can also be made subjected to the size the silicon wafer andthe available processing equipment.

FIG. 10 shows the peeling off of the soft mask patterned with Ag layer.It can be seen that the Ag film strongly bonded with the PDMS mask,indicating the successful creation of SAM on the Ag film and the Sisurface. A closer look at the microstructure pattern on the PDMS mask(FIG. 11B) illustrates that the micro-pattern on Ag film (FIG. 11A) wastransferred onto PDMS mask.

The fabricated PDMS soft mask may then be used to fabricate texturepatterns on the steel surface. Optical images of a PDMS mask withtextures and texture feature on a PDMS mask after the deposition of Agfilm are shown in FIGS. 12A and 12B, respectively. As shown in FIG. 12C,fabricated PDMS with Ag could act like a conventional photo mask withthe Ag film blocking UV light; therefore, outside the pattern feature, auniform photo resist covers the steel surface. It should be rioted thateven though the soft mask was wrinkled due to the difference in thematerial property between the PDMS and Ag, the pattern featurefabricated on the steel surface did not show obvious changes in size,pitch distance, and shape. It indicates that the wrinkles of the PDMSmask did not influence the photolithography process.

In an exemplary embodiment, the technique may be performed using asix-inch silicon wafer. FIGS. 13A and 13B show the large soft PDMS maskpeeled off from the six-inch Si wafer. A closer view of themicrostructure pattern on the PDMS mask, shown in FIG. 14B, indicatesthat the designed texture was transferred to the fabricated mask. Thetexture feature on the fabricated PDMS mask was the same with the mastertextural pattern on Ag film, shown in FIG. 14A. Feature size, pitchdistance, and shape remained unchanged.

In another exemplary embodiment, the soft mask technique andelectrochemical etching process may be performed on an engine component,i.e., piston ring surface. In an exemplary embodiment, a soft mask canbe fabricated according to the procedures discussed above. The detailedsteps are described as follows:

-   -   Si wafer cleaning procedure        -   a Si wafer is immersed in 5:1:1 of Deionized (“DI”) water:            NH₃OH:H₂O solution for 10 minutes at 75° C.        -   rinse with DI water        -   after alkaline rinse, the wafer is rapidly immersed in 5:1:1            of DI water:HCl:H₂O solution        -   rinse with DI water        -   immerse the wafer in 2% HF solution for 10 seconds        -   rinse with DI water an dry with a Semitool PSC-101 spin            rinse dryer    -   first SAM film: Dried Si wafer is immersed in a 1 mM        dodecyltrichlorosilane in acetone/DI water (5:1) solution for 24        hours    -   A 100 nm thick Ag film is deposited using Denton Infinity 22        E-beam evaporator at 9.0×10⁻⁷ torr    -   Surface texture pattern fabrication using conventional        photolithography procedure    -   Wet chemical etching of Ag film in 1:1 of HNO₃:DI water solution    -   Second SAM film: 16-mercaptoundecanoid acid molecules are        self-assembled onto the patterned Ag film    -   Polydimethylsiloxane (PDMS) is then spin-coated onto        SAM-patterned Ag surface to form a polymer film under a        temperature of 70° C. for three hours.    -   Peel off the patterned PDMS (soft mask) from Si wafer

The hydrophobicity of the modified surface with the first SAM film wasconfirmed with a 120° water contact angle (FTA32 goniometer). Thehydrophilicy of the modified surface with the second SAM film was alsoconfirmed with a water contact angle of 12°.

The PDMS soft mask can then be used to fabricate surface texturepatterns on engine piston ring surfaces. The piston ring sample may becleaned by steps with ethanol, acetone, and DI water, and then driedwith nitrogen. A positive photoresist film may then be applied on thecleaned surface by spin-coating process. The PDMS soft mask may then beused to cover the photoresist coated surface and exposed to the UV lightto transfer the surface texture pattern onto the photoresist film. ThePDMS soft mask may then removed from the surface after the UV exposure.After developing, the surface texture may be fabricated byelectrochemical etching process. TEM images of textured piston ringsurfaces are shown in FIGS. 15A and 15B.

In another aspect of this invention, a thin film layer, such as a DLCcoating, may be applied to the textured surfaces of engine or machinecomponents. There are several parameters to consider for such a thinfilm: (1) the nature and composition of the film; (2) the thickness ofthe film; (3) the adhesion characteristics of the film to the substrate;(4) the hardness of the film in relation to the substrate; and (5) thecompatibility of the film with lubricant chemistry.

Adding a film to a complicated textured surface may alter the dimensionand shape of the textured dimple. This may further alter the frictionreduction ability of the textures. If the thickness of the film is toothin, then the durability may be compromised. The nature and compositionof the film control the adhesion characteristics of the film to thesubstrate. It also requires the thin films to have the ability toconform to the textured surface with minimum residual stresses. Thereare no known films capable of meeting all these requirements.

To determine the sufficiency of different films, testing was performedas follows. A collection of high hardness wear resistant films weredeposited on textured surfaces, these include chromium nitride, titaniumnitride, carbides, and diamond-like-carbon films. The film thicknessranges from 90 nm to 200 nm thick. A test procedure was developed totest the degree of surface texture protection and the durability of thefilm. The four ball tester is used and the testing condition istabulated in Table 1.

TABLE 3 Tess condition for four ball tester Load Speed 5 Kg 3000 rpm(1.15 m/s) 10 Kg 2500 rpm (0.96 m/s) 15 Kg 2000 rpm (0.77 m/s) 20 Kg1500 rpm (0.57 m/s) 25 Kg 1000 rpm (0.38 m/s) 30 Kg 500 rpm (0.19 m/s)

The failure observed for thin films typically takes the form ofdelamination at the edge of the dimple (201) as shown in FIG. 16. Thefailure mechanism is primarily due to the high contact stresses at theedges of a dimple, especially under high-load, low-speed conditions.Under such stresses, the film covering the dimple typically tends tocrack. When shear stresses are also applied, the film tends todelaminate. Once the film delaminates, the film debris will act as anabrasive in the contact; hence, the friction will increase.

In view of the results of such thin film testing, a DLC film wasidentified as a preferred material to serve as a protection layerapplied to textured surface. An example of an embodiment of the DLC filmin accordance with this aspect of the present invention was tested. Thefilm thickness ranged from 90 nm to 250 nm and was deposited using aclosed field unbalanced magnetron sputtering ion plating system. Aninterlayer was engineered to increase adhesion between the steelsubstrate (for example, 52100 steel) and the DLC film. The configurationof the coating targets were Cr with 4.0 A current and C with 3.0 Acurrent. The deposition rates were 20.0 nm/min for Cr adhesiveinterlayer, 16.8 nm/min for CrC graded layer, and 10.1 nm/min for pureDLC top layer. The bias voltage varied from −60 V to −40 V during pureDLC deposition.

The DLC coating morphology and adhesion testing result is shown in FIG.17. FIG. 17A shows a SEM image of the coating morphology of coated (301)and uncoated (302) areas on AISI M42 steel substrate. FIG. 17B containsthe result of Rockwell indentation test at 150 kgf and shows no observeddelamination around the indent which indicates good adhesion of the DLCcoating to the substrate. The hardness of the DLC coating deposited atbias −40V, was measured on a thicker film (−2 micron thick) by Fischermicro-hardness tester at a load of 30 mN. The hardness of the coatingwas in a range of 12-18 GPa.

In another embodiment of this invention, modifications may be made tothe DLC layer that may be applied between moving surfaces of an internalcombustion engine over the textured layer. The technique describedherein allows for the DLC film to bring its own lubricants into theapplication it serves. While the internal combustion engine applicationis specifically discussed, those of ordinary skill in the art wouldrecognize that the film discussed herein may be similarly applied tomoving parts in other applications, including, for example, biomedicaldevices, telescopes, and gears. Traditionally, carbon has been verydifficult to lubricate, since it is essentially inert. In the techniquesdescribed herein, a process of molecular engineering films to obtainthat lubrication is described.

The process is first described generally, followed by more specificexamples below. DLC contains a carbon surface and is generally inert.Since only about 30% of the DLC surface is typically considered active,a first molecule is applied to the surface that reacts with that 30% ofthe carbon on the surface. Next, a second molecule is added by a coatingtechnique, which essentially floats on the surface. Then to prevent thematerial from evaporating, a polymeric monomer layer is added by a spraydeposition technique on top of the combination. Once the polymericmonomer is sprayed, it is activated by UV radiation. The polymericmonomer then becomes polymerized and essentially forms a tent or canopyon top of the surface. An effective thickness of the surface layer isapproximately 20 Angstroms. Moreover, 12-18 GPa has been found to be aneffective DLC hardness range for the application discussed.

Different type of molecules may be used as contemplated by thisinvention. For example, for applications such as friction reduction, ahydrocarbon with an alcohol functional group may be used as the firstmolecule. Then an inert functional molecule, such as Benzene, may beused as the second molecule. Then polystyrene film may be then appliedas the polymeric monomer layer on top.

With this coating, the DLC surface now acts in a manner like that of abumper. Traditional DLC film is hard, but it is also brittle. With thistechnique contemplated by this aspect of the present invention, the newlayer essentially acts like rubber, and the DLC becomes much more robustand useful. Benefits of the combination include the reduction offriction when it is used on surfaces of, for example, engine componentsas well as, for example, corrosion protection and rust prevention.

A more graphitic DLC film with a 150 nm thickness and 12 GPananohardness, contemplated by the present invention, was also tested bythe following procedure. DLC coated aluminum based disks (i.e., magnetichard disks for information storage technology) were used for chemicaldeposition, encapsulation, and thickness measurement, and were testedfor friction on a pin-on-disk tester. The chemical compounds weredeposited on DLC coated hard disks using a dip-coating technique.Solutions of 0.5% to 2.0% by weight of various compounds were preparedin cyclohexane or hexane. The solutions were mixed for a minimum of 10minutes in an ultrasonic bath prior to dip coating to ensure a completesolubilization of the compounds. Deposition was carried out in adipcoater using a dip speed of 40 to 60 mm/min. The resulting films werethen annealed at 165° F. for 10 minutes under argon atmosphere to bindthe molecules to the DLC surface while avoiding oxidation. Afterannealing, the films were washed with 10 ml of cyclohexane or hexane, toremove any molecules that were not chemically bound on the surface. Thefilm was then covered with a thin layer of polymeric monomers and thenUV irradiated to encapsulate the chemical film underneath.

The friction properties of both the encapsulated films and notencapsulated films were tested with the pin-on-disk machine using astep-loading procedure. The load was varied from 1 N to 25 N, and thespeed was varied from 0.015 m/s to 0.20 m/s to cover the hydrodynamicand boundary lubrication regimes. To extend the load range of the test,two diameters of the pin (balls) were used (1.5 mm and 6.125 mm). A steploading test was used to evaluate the deposited film. At each slidingspeed, eight different loads were applied and the steady state frictionfor that film was recorded. The testing results are illustrated in FIG.18 as the friction drops from boundary lubrication conditions(0.08-0.15) to 0.03, typical of hydrodynamic lubrication regime. Mixedcompounds (Comp A+Comp B+Comp C) appear to be able to control thefriction over the speed and load range much better than a singlecompound.

The durability of the encapsulated and not encapsulated films was testedusing a ball-on-three flats geometry of a four ball tester. The testingprocedure may be described as follows:

Stage I: Minicycle step loading sequence:

First cycle: started at 1.15 m/s linear speed (3000 rpm), increasedloads front 5, 10, 15, 20, 25, 30 Kg per every 3 minutes (or longeruntil a steady state friction trace is obtained). The typical time was 3minutes for each load.

Second cycle: lowered the speed from 1.15 m/s to 0.96 m/s (2500 rpm) andrepeated the loading from 5 Kg to 30 Kg steps.

For a total cycle, the speed changed from 1.15 m/s, 0.96 m/s, 0.77 m/s,0.57 m/s, 0.38 m/s, to 0.19 m/s in an increasingly severe testcondition. Minicycle 7 and 8 will basically repeat the condition ofcycle 1 and 2.

Stage II: Durability test (time to failure test sequence):

To clearly separate the various chemistries and textures, a time tofailure test sequence was used. After the eight mini-cycles of testing,if no failure was observed, then the test started at 1.15 m/s speed and2 kg load for 3 minutes, the load was increased to 5, 10, 20 Kg. At thattime, the test continued for one hour until failure. If no failure wasobserved, the load was increased to 30 kg for an hour, then 40 kg for anhour.

FIG. 19 depicts test results of various bonded chemical films undergoingthe cyclic step loading mini-cycles. The surfaces without bondedchemical films are as baseline cases. The baseline 1 (BL_1) and baseline2 (BL_2) were polished surfaces without surface texture. The baseline 3(BL_3) was textured with dimples. Simple circular dimples were used inFilm 5 and Film 6 and coated with bonded chemical films. No dimples wereused for Film 1 but bonded chemical film was applied. The baseline cases(BL_1 and BL_2) failed early as indicated by the upward arrows. All theother cases were survived the mini-cycle tests.

Following the above mentioned test procedure, if no failure wasobserved, the sample was subjected to stage II durability test asillustrated in FIG. 20. As can be seen from FIG. 20, a typical time tofailure is indicated by the friction trace showing a sudden increase offriction after a long steady friction level. The durability (or time tofailure) of the test cases are plotted in FIGS. 21 and 22.

The testing results of Stage I for a specific combination of chemistryand textures are summarized in FIG. 21. Baseline of polished surfacetested with paraffin oil failed early during the Stage I testing. Nofailure observed for baseline of polished surface tested with Mobil 1lubricant. No failure observed for the samples with bonded chemicalfilms.

The testing results of Stage II durability test for various bondedchemical films are shown in FIG. 22. After the samples have gone throughthe 7000 seconds of stage 1 testing, the test continues at 20 kg and3000 rpm speed for an hour. The baseline case using synthetic motor oilwithout bonded chemical film failed during the 30 kg load at about 13000seconds. Film 1 continued to 30 kg load for one hour, 40 kg load foranother hour, and 50 kg load for an additional hour without failure.Films 5 and 6 exhibited lower friction and appeared to be able tocontinue for much longer time. The concept of built-in bonded film hasbeen successfully demonstrated.

In a specific embodiment, the method for enhancing the DLC film mayinclude the following steps: (a) coating the DLC film with a mixture ofreactive chemicals specific to DLC surface such as long chain alcohols,olyel alcohols, glycols, polyglycols of various chain lengths; the filmis allowed to anneal at moderate temperatures (70° C. to 160° C.depending on the chemical species) for a period of time (5 to 30 minutesdepending on specific molecular structure); (b) solvent washing afterannealing to remove any unreacted molecules; (c) addingalkylcyclopentanes or other inert molecules to the surface by dipcoatingto fill the unreacted space on the surface; (d) adding polymericmonomers to the top of the surface using spin coating or other means;and (e) radiating the layer of polymeric monomers with UV light ofsuitable wavelength and time duration to encapsulate the molecularassembly underneath.

Other molecules that may be used separately or in combination withalcohols are Amine O, thiocarbamates, esters, and polyesters, copperoleates, and sulfur leates. Under typical conditions, about 20% to 40%of the DLC surface will be covered with a monolayer or more of thesemolecules after washing with appropriate solvents. The polymeric monomermay be comprised of polystyrene or polypropylene or other polymer blendssuitable for UV irradiation induced polymerization.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

1-27. (canceled)
 28. A surface protection layer of a component comprising: a patterned texture on a surface of an engine component; a thin film that covers the patterned texture; and a bonded chemical film that covers the thin film.
 29. A surface protection layer according to claim 28, wherein the thin film is a diamond-like-carbon film. 30-31. (canceled)
 32. A method for enhancing a diamond-like-carbon film comprising: coating the diamond-like-carbon film with a monomer layer; annealing the monomer layer with the diamond-like-carbon film to form a chemically bonded film; covering the chemically bonded film with a layer of polymeric monomers; and radiating the layer of polymeric monomers with UV to encapsulate the film underneath.
 33. A method according to claim 32, wherein the friction coefficient of the diamond-like-carbon film is reduced.
 34. A method according to claim 32, further comprising: applying the diamond-like-carbon film to a surface of an internal combustion engine.
 35. A method according to claim 32, wherein the polymeric monomer is comprised of polystyrene.
 36. A method for enhancing a diamond-like-carbon film comprising: (a) coating a diamond-like-carbon film with a mixture of reactive chemicals; (b) annealing the diamond-like-carbon film; (c) applying a solvent wash to the diamond-like-carbon after annealing; (d) applying inert molecules to the surface of the diamond-like-carbon film by dipcoating; (e) adding polymeric monomers to the top of the surface of the diamond-like-carbon; and (f) radiating the layer of polymeric monomers with UV light.
 37. A method according to claim 36, wherein the friction coefficient of the diamond-like-carbon film is reduced.
 38. A method according to claim 36, further comprising applying the diamond-like-carbon film to a surface of an internal combustion engine, wherein the diamond-like-carbon film has a nanohardness of substantially between 10 GPa to 20 GPa. 39-40. (canceled) 